Spare cash finally found in can (4)
I believe the answer is:
thin
'spare' is the definition.
(similar in meaning)
'cash finally found in can' is the wordplay.
'finally' suggests the final letters.
'found in' indicates putting letters inside.
'can' becomes 'tin' (both are metal containers).
The final letter of 'cash' is 'h'.
'h' put inside 'tin' is 'THIN'.
(Other definitions for thin that I've seen before include "Flimsy" , "Unconvincing" , "Narrow, slim" , "Carrying little fat" , "(Of smile) weak, forced" .)